This invention relates to a process for the production of a gas containing sulfur dioxide from an aqueous ammonium sulfate solution obtained as a by-product, in particular, during the synthesis or organic compounds, such as caprolactam, for example.
The synthesis of several organic compounds, especially that of .epsilon.-caprolactam by way of cyclohexanone oxime, necessarily yields large amounts of ammonium sulfate, the precise amount of which is dependent on the method, for example, approximately 1 to 4 tons per ton of caprolactam. This ammonium sulfate is of very little economic value in the production of these organic compounds, since ammonium sulfate can be utilized only to a limited extent. A direct use as a fertilizer is impossible, because the impurities contained in this product result in a poorly crystallized, discolored ammonium sulfate. To attempt to upgrade the value of the ammonium sulfate, it is known to subject the ammonium sulfate solution obtained during the production of caprolactam, prior to crystallization, to a thermal pressure treatment, in order to produce a fertilizer (DAS [German Published Application] 1,284,954), or to treat this ammonium sulfate solution with aluminum sulfate and the sodium salt to ethylenediaminetetraacetic acid (Italian Patent No. 678,180). Ammonium sulfate solutions rich in organic components, obtained in the caprolactam production, can be evaporated and thereby separated from the organic components and can then be caused to crystallize, in accordance with Dutch Patent Applications 65 16058 and 65 16059. All of these measures, though increasing the quality of the thus-produced ammonium sulfate, are nevertheless relatively futile from a realistic marketing viewpoint because the use of high grade sulfate as a fertilizer is presently possible only to a very limited degree.
It is furthermore known from DOS [German Unexamined Published Application] 1,916,149 to neutralize the mixture obtained during the rearrangement of cyclohexanone oxime with sulfuric acid with the use of a metallic oxide in place of ammonia; to reductively split the thus-formed metallic sulfates to the metallic oxides and sulfur dioxide; to re-use the metallic oxide for the neutralization; and to process the sulfur dioxide into sulfuric acid, which latter is again utilized in the rearrangement process. The method has the disadvantage that the neutralization with metallic oxides takes place more slowly than with the use of ammonia, and that the universally employed neutralization with ammonia must be abandoned.
Finally, it has been suggested in U.S. Pat. No. 3,795,731 to combust solid, finely divided ammonium sulfate at temperatures of between 800.degree. and 1250.degree. C. to sulfur dioxide, steam, and nitrogen, and to maintain in the gases exiting from the combustion chamber, an oxygen concentration of between 1% and 10% by volume. In order to obtain the solid ammonium sulfate for the combustion, the salt must be crystallized out of the primarily obtained ammonium sulfate solution. This crystallization is generally conducted in an expensive, three-stage crystallizer and requires considerable expenses due to steam consumption and maintenance of a vacuum.
This invention is based on the problem of simplifying the recovery of the sulfur dioxide from ammonium sulfate solutions obtained in the synthesis of organic compounds by combustion, and to improve this process with respect to economy. In particular, the expensive crystallization of the ammonium sulfate in crystallizers prior to combustion is to be eliminated.
Thus, a principal object of this invention is to provide an improved process for the production of SO.sub.2 -containing gases from aqueous solutions of ammonium sulfate. Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.